112 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
fixed screw, adjustable, however, for final adjustment. The end of the 
micrometer screw is milled, but of a small diameter, so that no force can 
be exerted to endanger the cover- glass. Fixed on the screw between 
two nuts is a brass drum with a 1/2 in. face ; a knife-edge index-finger 
is fixed to the top of the stand, and projects over the top of the drum ; 
to the outside diameter of the drum is fixed a strip of glazed paper 
Fig. 8. 
Actual size. 
provided with a series of divisions. The first gives the thickness of 
cover-glass in thousandths of in., the second in hundredths of mm. 
The third indicates the proper tube-length with various thicknesses of 
cover-glass with a non-adjustable 1/4 corrected under a tube-length of 
8^ in., and cover thickness of O' 16 mm. ; the fourth gives the tube- 
length of a 1/5 in. objective under the same conditions; the fifth for a 
1/8, and the sixth for a 1/12 for same conditions of tube-length and 
cover ; the seventh is for a 1/6 with the same cover, and tube-length 
of 160 mm. 
In objectives provided with cover correction the graduation is so 
arranged as to read to 0* 01 mm. No matter what the power of objective 
or whether dry or water-immersion, the number gives proper correction 
for a thickness corresponding to it. Thus, with a cover-glass of 
